1. Field of the Invention
This invention relates to computer circuitry and, more particularly, to apparatus for eliminating reverse bias breakdown in bi CMOS tristate driver circuits.
2. History of the Prior Art
Tristate driver circuits are used to place signals on the bus of a computer, among other things. In this application, these circuits provide a first voltage equivalent to a binary one condition, a second voltage equivalent to zero condition, and an open circuit condition. These circuits are used because it is desired when many different components are connected to a bus that the active driver circuits be able to produce the two distinct binary values for driving the bus but that the inactive driver circuits appear as though they do not exist as a bus connection when they are not driving the bus.
A significant problem of these circuits when formed in bi CMOS material is that significant voltage conditions on the bus may back bias the output transistors of the tristate drivers. If the back bias is large enough, these transistors may fail; if the voltage is less but is repeated enough times over a short enough period, the operation of the transistors may be degraded and they may ultimately fail.